This invention relates to a technique for measurement of the liquid water content of clouds or fog, using a hot-wire sensor; and also calibration of the system.
The liquid water content of fogs and clouds is still one of the more fundamental variables that cloud physicists attempt to measure. Its distribution and evolution are relevant in cloud seeding, cloud modeling and cloud microphysics. Of the instruments currently available to measure liquid water, the John-Williams hot wire has been the most widely used. In this instrument a constant current passing through a wire exposed to the airstream heats the wire, and its temperature is monitored to give a measure of the liquid water. A similar wire which is kept dry is used to compensate for the heat supplied to the air moving past.
Merceret and Schricker, in the Vol. 14, April 1975 issue of the Journal of Applied Meteorology, pp. 319-326 describe a "nimbiometer" in which a hot wire is maintained at a constant temperature and the power monitored. Although this more readily permits analytic treatment, this instrument, like the Johnson-Williams, still requires either a wet wind tunnel or another liquid water instrument for calibration. Devices which yield the full droplet size distribution, such as the Knollenberg optical scattering probe or the older soot slides are also used to determine liquid water by integration of the droplet spectrum. The former is expensive, technically sophisticated, and requires computer backup to provide a real-time output of the liquid water, while the latter is labor-intensive in postflight analysis and also discriminates against the larger droplets because of its small sample volume.
King et al in an article in the December 1978 issue of the Journal of Applied Meteorology, pp. 1809-1813, describe "A Hot-Wire Liquid Water Device Having Fully Calculable Response Characteristics" that is robust, requires at most a simple dry calibration, has a stated sensitivity of 0.02 gram/cubic meter, a response time of the order of 0.05 seconds and an accuracy of about 5 percent at 1 gram/cubic meter (more dense than thick fog).
Determination of electro-optical (E-O) systems performance under low visibility conditions such as fogs and hazes is currently a problem whose solution is among the top priorities. A relationship is being sought between the propagation properties of the visible or infra-red radiation and meteorological parameter(s). For fog, this has been done in theory using liquid water content, i.e. mass density of the atmospheric water droplet content. This has caused very high level of interest relative to measurement systems for liquid water content. Meteorologists have stated that they have no satisfactory liquid water content (LWC) systems for ground based use.